The conventional combustion process in a diesel engine is initiated by the direct injection of fuel into a combustion chamber containing compressed air. The fuel is essentially ignited instantaneously upon injection into a highly compressed combustion chamber, and thus produces a diffusion flame or flame front extending along the plumes of the injected fuel. The fuel is directly injected into the combustion chamber by a fuel injector having a perforated nozzle tip extending into the combustion chamber. The nozzle tip may extend slightly into the combustion chamber from a wall of the chamber located opposite a reciprocating piston of the combustion chamber. The nozzle tip comprises a series of holes from which fuel is extruded into the combustion chamber.
Nozzle tips are commonly designed with the largest number of spray holes having the smallest diameter possible to inject the required fuel quantity at each locomotive engine's speed and load setting in order to preserve the nozzle's wall thickness and strength.
Diesel fuel is injected at 10,000-18,000 psi out through 5 to 7 individual, separate and distinct round orifice holes located at the very bottom of the spray tip. The resultant spray pattern consists of separate and distinct solid plumes of atomized fuel droplets that evaporate and mix with the combustion air at the proper temperature, pressure and air/fuel ratio auto-ignite.
The spray holes are the first components of the injector assembly to ultimately affect the service life and durability of the nozzle assembly. Typically, a spray tip's fuel flow characteristics deteriorate approximately 0.5% per year for the first three years, increasing rapidly to 1% to 1.5% per year by a service life of 5 and 6 years. The service life and long term durability of the whole geometry is strictly a function of the total fuel consumed or injected through each of the spray holes.
The fuel plumes after an extended service life become less of an atomized and turbulent gaseous spray pattern and become more like a solid laminar fuel stream taking more time to break up into small droplets, evaporate and burn. The ignition of a gaseous spray is much more efficient than that of a solid stream of fuel.
In an effort to overcome such short comings, spay tip orifices require very precise machining at great expense. The manufacture of such spray tips requires the creation of spray holes, which must be perfectly round having a precise diameter in order to provide the correct geometry for the proper fuel flow and atomization. Such spray holes require a certain case hardness depth to help prevent premature wear and erosion. This hardening process is very critical. If the case depth is too shallow, the hole will wear prematurely and if too hard will become brittle, the hole will be prone to stress fractures and premature failures.
Thus, what is needed is a method and apparatus capable of providing a durable spray assembly that can deliver a consistent atomized spray plume over the service life of a fuel injector.